Electrical surface heating systems are well known and have been used for heating ceiling, wall and floor surfaces in rooms, as well as outdoor surfaces, for many years. These systems generally involve the placement of one or more lengths of electrical conducting material beneath and in contact with the surface to be heated. By transmitting electrical current through the conducting material, heat is generated and then transferred to the contacting surfaces.
The use of electrical surface heating systems in floors has numerous advantages over other types of heating systems such as base-board or forced air systems. For example, the heat from base-board and forced air heating systems begins traveling upward by means of convection as soon as it is introduced into the room interior. Thus, at positions remote from the heat source, very little warm air remains near the floor surface while the ceiling area is quite warm. In fact, with many base-board and forced air heating systems the air temperature at the ceiling may be as much as 40.degree. F. warmer than the air temperature at the floor. Thus, if one wishes to maintain air temperatures near the floor at a comfortable level, the base-board or forced air heaters must introduce heat in quantities which produce uncomfortably high temperatures near the ceiling. This heat continues to travel upwards, passing through the ceiling and through the roof to the outside atmosphere. It is clear that such a situation is energy inefficient and, therefore, undesirable in light of present attempts both to conserve energy resources and to reduce heating costs.
In contrast, with essentially uniform heating of the floor surface a much lower temperature gradient is experienced between the floor and ceiling of a room. In fact, temperature differences of as little as 3.degree. F. between the floor and the ceiling of a room are easily achievable in normal home operation of a system which provides uniform heating of the floor surface. This is because the heat is emitted uniformly across the floor surface and then permeates through the entire room space above the floor as it travels upward to the ceiling. In addition, because of the uniform room heating, the floor temperature need only be slightly higher (i.e. one or two degrees Fahrenheit) than the desired room air temperature in order to maintain that desired temperature.
Numerous problems have been encountered in past attempts to obtain uniform heating of floor surfaces by means of conductors embedded therein. For example, if the conductor is not in contact with all the surfaces to be heated, the heat must be transferred from that portion of the surface which does contact the conductor to those surface areas which do not contact the conductor. Such a transfer involves large thermal gradients or temperature differences across the floor surface and thus, in order to maintain a desired average floor temperature, the floor surface in contact with the conductor must be raised to a temperature which may be much higher than the desired average temperature.
In cases where conductors are spaced too far apart, the conductor temperatures necessary to achieve the desired average floor temperature are sufficiently high that "hot spots" are created which may cause scorching and fire damage to the floor surface or damage to heat sensitive materials positioned upon the floor near the conductor.
The "hot spot" problem is not adequately overcome by running a single conductor back and forth through the floor along close parallel paths, since voltage drop along the length of the conductor increases to unacceptable levels as conductor length increases. In attempting to overcome this problem by other means, metallic strips have sometimes been placed in the floor so as to contact much of the floor near its surface. Although this prior art system experiences a low overall voltage drop, numerous problems have been experienced both in the installation and in the operation of such metallic strips. For example, expansion of the metallic strips during heating, or movement of the strips in response to people walking on the floor, may cause crinkling noises or other types of noises which are clearly audible from the floor surface.
Such problems are particularly common when the metallic strips are very thin in construction. In addition, the very thin conductors break easily, are susceptible to being cut during installation, pose a hazard to the installer due to their sharp edges, and are also susceptible to being broken after installation if sufficient floor movement is experienced (i.e. movement such as that caused by moving large furniture across the floor). It is well-known that extremely thin strips of metal conductor have a small overall electrical resistance, and thus they require very low current levels to achieve the necessary heating. In light of this, even in the presence of the above-described problems, use of the thinnest metallic strip available continues to be preferred.
Although use of such thin metallic strips reduces energy requirements, truly uniform heating of the floor surface with those very thin strips has not been possible. This is because metallic strips constructed of popular heating materials such as brass cannot be reliably manufactured to uniformly maintain a truly useful thinness of less than about 0.002 inches. As a result of this non-uniformity, uneven heating of the conductor is experienced. Thus, in order obtain uniform heating, the metallic strips are typically thicker than would otherwise be desired, requiring a correspondingly higher current level for producing the necessary heat.
Other attempts to obtain uniform heat distribution in floor surfaces have involved the use of hardware cloth or other wide mesh metallic materials as conductors. Unfortunately, these wide mesh arrangements have also required excessive conductor temperatures in order to obtain acceptable average surface temperatures. As a result of the high conductor temperatures, floor surfaces and materials such as carpeting positioned adjacent to the conductors may eventually become marked with the pattern of the mesh conductors.
In systems using metallic strips or hardware cloth type heating elements, problems are encountered when it becomes necessary for the heating element to fit around pipes or other fixtures. If the heating element is cut in order to make this fit, the current that normally goes through the cut portion has to travel through an adjacent portion until it reaches the other side of the cut. The increased current flow through the uncut sections causes hot spots with their attendant fire danger. Problems are also encountered where the heating element defines a corner since the electrons traveling on the heating element try to take an "inside track" and mostly pass on the inside portion of the heating element as they round the corner, thus causing additional hot spots.
For example, in a heating system utilizing a wire mesh heating element having 216 small gauge longitudinal wires and having a typical resistance of 0.001262 ohms, and with the heating element extending about 300 feet in length and operating at a typical level of 89 amperes, each wire of the heating element carries about 0.412 amps. If 196 of the adjacent wires were cut, the resistance of the mesh would not increase significantly and therefore, the remaining 20 adjacent wires would have to carry almost the full 89 amps around the cut. This would mean that each wire would have to carry about 4.45 amps which would cause the small gauge wires to become very hot. In typical wire mesh heating elements this situation would constitute a fire hazard to flammable materials positioned near the uncut adjacent wires. If an additional 10 wires were cut, the remaining 10 wires would carry about 8.9 amps each, producing heat levels in excess of the flash point of many materials. Although such a condition can exist in many prior art heating systems, no means has previously been available for preventing such increased current flow through portions of the heating element adjacent cuts or adjacent inside corners.
In each of the prior art systems described above, as additional heating is called for, voltage through the conductor must be increased. As voltage levels increase, the risk of fire or electrical shock also increases. This condition poses a particular danger to individuals who are laying carpet or fixing articles to a floor by means of screws or nails, or who are otherwise exposed to contact with the conductors.
Although hazards due to electrical shock and fire are present, the previous devices have not provided adequate protective devices to overcome these dangers. For example, when increased conductor heating requirements are encountered, transformers may overheat and become a potential fire hazard. Because of the excessive cost of presently available protective devices, typical prior art systems have not provided any means of recognizing and overcoming transformer overheating problems.
In addition to dangers from over temperature conditions, low voltage electrical systems also pose a great hazard during over-current and under-current conditions. When a short circuit occurs in a low voltage electrical system, high levels of current are transmitted therethrough. However, the protective devices on the typical 240 volt home power supply will very likely not even respond to currents of a magnitude which will do damage in the low voltage situation. Further, even if the current level in the low voltage system is so high as to be detectable by current protection devices on the 240 volt system, the normal time delay of such devices often allows the high current flow to continue for several minutes before the circuit is disconnected. The time delay before disconnection is generally so long that extremely high temperatures in excess of several hundred degrees Fahrenheit may easily be reached in the low voltage heating system.
In situations where the conductive surface becomes ruptured or otherwise experiences an open circuit condition, very little current will be transmitted into the system but extremely high voltage levels may be experienced across the ruptured location. These high voltage levels can result in arcing, which creates a serious risk of fire since temperatures in the arc may exceed several thousand degrees Fahrenheit.
Although the above dangers are often present in prior art electrical heating systems, devices for providing over-current and under-current protection are not practically available for use on low voltage systems. Typical devices used for providing such protection are designed for use in applications where the operating level voltage is in excess of 240 volts. The cost of these devices is sufficiently high that, even if they could be adapted for use in low voltage applications, their use would simply make such systems so expensive as to remove then from consideration as a viable alternative for home heating.
In addition to the other problems, transformers and control equipment in low voltage electrical surface heating systems have previously been directly exposed to possible damage upon their connection to A.C. power systems. If connection of the low voltage system to an A.C. power source is made at a time when the polarity of charge in the transformer coils is significantly out of phase with the A.C. system, a power surge may be experienced through the electrical circuitry which can cause damage to transformers as well as to associated electrical control equipment. This surge can be so large as to produce a jarring shock which is easily audible and may be physically felt by one positioned within the room to be heated. This situation has constituted a substantial stumbling block in previous attempts to provide low-voltage electrical surface heating systems which could be connected directly to an A.C. power source.
Prior art surface heating systems have often been utilized for heating exterior surfaces such as driveways in order to prevent the formation of ice thereon. However, operation of the majority of those systems has required manual initiation with operation continuing until manually terminated. This has made the use of such systems economically unattractive if not unfeasable in many instances. For example, if snow is expected during the night, the user must either periodically get up to see if snow is falling or else turn the system on before retiring to bed. If snow does not come and if the user does not awaken and turn the system off, rather large amounts of energy are wasted in unnecessarily heating the exterior surface. Such waste becomes very noticeable upon arrival of the user's monthly electric bill. Attempts have been made to automate the operation of such systems by utilizing moisture detectors to turn on the system. However, although moisture may be detected so that heating is initiated, if air and surface temperature conditions are not right, ice will not form in any event and energy is again wasted. In addition, moisture detectors used in such attempts have typically been designed for positioning away from the surface to be heated, and thus they could detect moisture and turn the heat on even when the surface to be heated was dry.
In light of the above considerations it would be a great improvement in the art to provide an electrical heating system which could be embedded in floors or similar surfaces so as to provide essentially uniform heating across the associated surface while minimizing the amount of electrical power required to achieve the desired heating. A further improvement in the art would be to provide such a heating system which could be installed and operated with very little possibility of malfunction, failure, or fire hazard due to electrical shorting or physical breaking of the heating element.
Still a further improvement in the art would be to provide such a heating system which would operate at low voltage and low current levels and which would pose essentially no hazard of shock to persons contacting the heating element surface. It would also be an important improvement in the art to provide such an electrical heating system which could be connected to an A.C. voltage source in a synchronized manner so as to avoid power surges. Still another important improvement in the art would be to provide such an electrical heating system which includes protective devices for preventing fire or other damage due to transformer overheating or due to over-current or under-current conditions in the heating system. A still further improvement would be to provide such a heating system which could automatically prevent formation of ice on exposed surfaces in an energy efficient and economical manner.